This invention relates to load sensing devices designed to detect the quality level of the machining process during the machining operation, and more specifically, to sensors or transducers used in such devices for monitoring applied torque and axial loading.
In a recently developed method for detecting the quality of the machining process during machining operations, the torque and axial thrust applied either to the tool, part or holder is sensed during the machining operation, and an electrical signal emitted to determine the "signature" of that operation. The "signature" is compared with signatures of previously machined parts to determine quality level limits for each segment of an operation. The quality of parts machined has a direct correlation to the quality of the machining process.
For example, a first set of sensors mounted to a drill chuck indicate the axial thrust of the drill against the workpiece, while a second set of sensors also mounted to the chuck indicate the torque developed between the drill and the driving source during the various segments of the drilling process.
The drilling process may be conveniently divided into four segments: the first occurring prior to applying the drill to the workpiece, the second occurring as the drill enters the workpiece, the third occurring during the drilling operation, and the fourth occurring as the drill breaks through the workpiece upon completing the drilling operation.
Minimum and maximum limits for torque and axial force for each segment may be determined from prior satisfactory drilling operations. These limits provide comparison guides which indicate whether or not a particular operation is being performed properly at the time it is being carried out. Defective workpieces, defective tools, and malfunctioning machinery including such components as bearings and cams are indicated immediately; and, this indication can be used to actuate an automatic shutdown, an alarm, or other means of alerting the operator to prevent further damage to parts, tools or machinery. This method provides an additional advantage in that the degradation of the tools and machinery as well as the material composition of stock can be monitored to provide an alert that corrective action is required prior to the occurrence of any damage.
To further illustrate the process by which a machining operation "signature" is obtained, reference is made to FIGS. 5 through 8. These Figures disclose a workpiece 510 positioned to have material removed therefrom by a drill 512. The drill 512 rotates a drill bit 514 in a well known manner and the drill bit 514 is disposed so as to penetrate and pass through the workpiece 510 upon relative rotation of the bit 514 and the workpiece 510. The transducer 516 is associated with the drill 512 to sense the torque required to effect rotation of the bit 514 as it passes through the workpiece 510. The torque and force signal monitored by the transducer 516 are directed along the line 520 to a programmable controller 522. A linear variable differential transformer 524 is associated with the drill 512 and provides an output signal on line 526 which is directed to the programmable controller 522 and which is indicative of the position of the drill 512 with respect to the workpiece 510. The linear variable differential transformer 524 includes a probe 530 associated therewith which physically contacts a surface of the workpiece 510 and provides a variable output signal to the programmable controller 522 which is indicative of the relative position of the cutting tool 514 with respect to the workpiece 510. Power supply 532 is provided for energizing the programmable controller 522 in a well known fashion.
In the example ilustrated in FIG. 5, for the drilling of a hole in a wall 518 of the workpiece 510, the torque required to effect rotation of bit 514 will be very low prior to the bit 514 engaging the wall portion 518 of the workpiece 510. As the bit penetrates the wall 519, the torque required to effect rotation of the bit 514 will increase to a higher, relatively constant level until the bit 514 starts to exit from the wall portion 518. At that time the torque required to effect rotation of the bit 514 will drop substantially.
By sensing the torque required to effect the rotation of bit 514 as the bit penetrates and passes through the wall portion 518, various quality characteristics of the workpiece 510 can be determined to determine whether the workpiece 510 is an acceptable workpiece meeting predetermined quality standards. For example, the thickness of the wall 518 can be sensed or the hardness of the material forming the wall 518 can be sensed. "Unusual" signatures not explained by the condition of the tool or workpiece are generally indicative of machine wear or failure. The "unusual" signatures can be catalogued to aid in predicting the need for machine maintenance.
Programmable controller 522 is utilized to generate a torque versus distance "signature" curve as is illustrated in FIG. 6. The torque distance curve plots the torque sensed by the transducer 516 as the ordinate against the particular distance, as the abscissa, that the drill 512 travels as it is sensed by the linear variable differential transformer 524. When the drill 514 passes through the wall portion 518 of the workpiece 510, the drill 514 and the workpiece 510 must move relative to each other through a predetermined cycle. This cycle will include the drill approaching the workpiece 510, the drill penetrating the workpiece 510 and then the drill exiting the workpiece 510 as it passes through the wall portion 518. One such complete movement of the drill 514 relative to the workpiece 510 is defined as a cycle. Each cycle will be broken into a plurality of increments during which torque sensed by the transducer 516 is measured.
FIG. 6 illustrates a typical torque versus distance curve for the drilling operation illustrated in FIG. 5. As the drill bit 514 approaches the wall portion 518 of the workpiece 510, no torque will be exerted on the drill bit 514 by the workpiece 510 and thus the portion 640 of the curve will be generated which is indicative of no torque being exerted between the workpiece and the drill during the initial movement of the drill toward the workpiece. Upon initial engagement of the bit 514 with the wall 518 of the workpiece 510, the torque will rapidly rise as indicated by drawing numeral 642. This function is known as leading edge turn on and can be utilized by the programmable controller 522 to locate the exact distance that a bit 514 has travelled before it has engaged the wall 518 of the workpiece 510. As the bit 514 penetrates the wall 518, the torque exerted will be relatively high but somewhat constant as exhibited at 644 in FIG. 6. When the bit 514 breaks through the surface 521 of the wall 518 of the workpiece 510, the torque exerted on the bit 514 will drop as exhibited at 646 in FIG. 6. This function is known as trailing edge turn off.
FIG. 7 illustrates another example of a drilling operation wherein like parts are identified by like numerals. In this example, it is desired to drill a cross hole 760 into an existing cavity or cross port 762. As the drill bit 514 approaches the side surface 780 of the workpiece 510, initial contact establishes a sharply rising torque curve as shown by drawing numeral 872 in FIG. 8 for "leading edge" electrical switching or turn-on. Upon penetration of the drill bit 514 into the workpiece 510, the torque curve will flatten, as illustrated in FIG. 8 by drawing numeral 874, for the portion of the torque-distance curve which is indicative of high torque; and torque remains relatively constant until the drill bit 514 enters the existing cavity 762. Upon entering the existing cavity 762, the amount of torque required for the drill bit 514 to rotate will be decreased and the torque-distance curve will dip as indicated by drawing numeral 876 in FIG. 8.
If it is desired to further penetrate the wall 782 of the workpiece 510, a torque-distance curve shown in FIG. 8 will appear as indicated by drawing numeral 878 as the bit 514 further penetrates the wall 782.
The portion of the curve in FIG. 8 designated by drawing number 878 will be indicative of high, but relatively constant torque as the drill bit again breaks through the wall portion 782. The torque required to rotate the drill 514 will drop, at the point indicated by drawing numeral 881 in FIG. 8, and "trailing edge" electrical switching or turn off will have been accomplished.
Prior to the development of the signature quality control method, most monitoring of machining operations was designed to minimize damage occurring when a tool failed. An example of such a prior art system is provided in U.S. Pat. No. 3,836,834. This patent described the sensing of bending and feed forces on a machine tool which are compared to limits. If a limit is exceeded, the machine is shut off. There is not attempt in the '834 patent to detect tool or machine degradation, nor to sense additional parameters such as torque or vibration for diagnostic purposes.
Despite the advantages of the signature quality control method described above, there remain a number of practical problems in implementing this method. One set of problems is centered around sensing the torque and lateral thrust encountered in the machining operations. As shown in FIG. 1, one method of sensing both torque and lateral thrust applied to a tool 101 is to mount one strain gage 102 aligned with the longitudinal axis 103 of the holder 104 to sense axial or lateral force and a second strain gage 105 positioned at 45 degrees to the first to sense torqe by sensing shear strains in the plane of maximum shear as is well known to those skilled in the art of stress-strain measurement. Unfortunately, where the tool is rotating, this technique requires two coupling devices, such as rotary transformers or slip rings, for connection to the individual strain gages to extract the information necessary to produce a desired machining "signature". This double coupling requirement is costly and cumbersome, and accordingly has precluded its use in most practical applications.
Where the tool is held stationary, and the work is rotated or otherwise moved with respect to the tool, the rotary coupling devices may be eliminated, but a number of serious practical problems remain. Two signal conditioners, bridge networks and displays or other indicating devices and two sets of quality limits are required to read out the information provided by the sensors and to control the machining process. In addition, these limits and displays must correlated to establish a satisfactory combined quality criteria and means for controlling each phase of a machining operation.
A further set of problems are related to the resistance element in electrical resistance strain gages which are sometimes used as force sensors in machining operations. These strain gage sensors generally do not produce sufficient output when applied to machining tools or machine tool holders to permit reliable detection. One approach intended to overcome this problem is to substitute piezo electric elements have not met with widespread success because of their sensitivity to temperature and inability to reliably withstand the vibration encountered in machining operations.